The invention relates to an additional electric heater, in particular for a heating or air-conditioning system of a motor vehicle.
In low-consumption vehicles the low level of heating which is available makes it necessary to provide additional heating power to heat up the passenger compartment and to rapidly remove precipitation (ice or water), in particular from the windshield. To do this it is known, in the case of heat exchangers which are constructed from flat tubes through which a heat transfer medium flows which outputs heat in the case of heating, to provide an additional heating means in the form of PTC heating elements at least on the outermost tubes, these elements being generally ceramic PTC assemblies which usually have a surface temperature of between 110 and 160° C. irrespective of the peripheral conditions such as applied voltage, nominal resistance, air flow. Owing to restrictions in the shaping and geometry, the mounting or the design of an additional electric heater is extremely costly. In addition, the ceramic PTC assemblies are relatively heavy.
WO 03/098124 A1 discloses a heat exchanger with an additional electric heater having a plurality of flat tubes which are arranged parallel to one another and through which a heat transfer medium flows, wherein at least some of the flat tubes are assigned a heating element as an additional heating means which is mounted after the heat exchanger has been soldered, is operated electrically and is mounted by means of a securing element on the end side of the heat exchanger with respect to the corresponding flat tube and so as to extend parallel to the latter by means of the securing element which also extends parallel to the flat tube. In this context, a polymer PTC material which is composed of a film and is placed around a central electrode in a U shape can be used, said material just filling, with a slight pressing tension, the space within the securing grill. In this context, the securing grill is at an intermediate potential and has to be electrically insulated from the heating element. However, such a configuration is not entirely satisfactory.
In addition, DE 10 2004 027 687 A1 discloses an electric heating device having a plurality of heating element panels which are arranged parallel to one another in order to define an air duct between two adjacent heating element panels, a positive electrode element which is connected to one end side of each heating element panel and a negative electrode element which is connected to the other end side of each heating element panel. The heating element panels are, for example, electrically conductive resin into which an electrically conductive filling material is mixed. In this context, the electrically conductive resin generally has a positive resistance/temperature characteristic curve in which the electrical resistance rises at a predetermined temperature or above said temperature. The current flows here from one electrode element through the heating element panels to the other electrode element.